***neither a teacher nor a native-speaker***
Here are two imaginary dialogue I wrote to examplify the difference. There would be other different places to use them.
P1: We need to change the schedule.
P2: Depending on what?
P1: Will we able to go on a vacation in this summer?
P1: On what?
P2: On whether we could save enough money or not until the summer comes.
Student or Learner